Color images are commonly obtained in the silver halide photographic art by reaction between the development product of a silver halide developing agent (e.g., oxidized aromatic primary amine developing agent) and a color forming compound commonly referred to as a coupler. The reaction between the coupler and oxidized developing agent results in coupling of the oxidized developing agent to the coupler at a reactive site on the coupler, known as the coupling position, and yields a dye. The subtractive process of color formation is ordinarily employed in color photographic elements, and the dyes produced by coupling are usually cyan, magenta, or yellow dyes which are formed in or adjacent to silver halide emulsion layers sensitive to red, green, or blue radiation, respectively.
Couplers well known for forming magenta dyes include, e.g., pyrazolones and bicyclic pryrazoloazoles, as described, for example, in U.S. Pat. Nos. 3,725,067; 3,810,761; 4,443,536; 4,540,654; and 4,621,046.
However, such known couplers often have drawbacks.
One such drawback of many magenta-dye-forming couplers is that they are not as efficiently reactive with oxidized developing agent as desired, such that the magenta dyes formed from such couplers in silver halide photographic elements yield relatively low levels of maximum density (Dmax) and/or contrast.
A further common drawback of many magenta-dye-forming couplers is that the magenta dyes formed from such couplers have poor light stability, such that the dyes fade too quickly when exposed to daylight. Photographic elements containing such imaging dyes can exhibit an unacceptable decrease in absorption of green light too quickly, resulting in color images that appear too green.
Another con, non drawback is the relatively high equivalent weight of many magenta-dye-forming couplers. The term "equivalent weight", as used herein is equal to the molecular weight of the coupler divided by the number of efficiently reactive coupling moieties in the coupler molecule. Each efficiently reactive coupling moiety is capable of reacting with oxidized developing agent to form a colored dye moiety. The higher the equivalent weight of the coupler is, the larger is the mass of coupler that must be included in a photographic element layer in order to be able to produce the desired amount of developed image dye optical density. The need for a larger mass of coupler in a layer results in a thicker layer, which inherently reduces the transparency and optical sharpness of the layer. Thus, lower equivalent weight couplers allow for thinner, more transparent, optically sharper layers. Unfortunately, the overall mass of a coupler molecule must be relatively large in order to provide sufficient organic ballast to properly suspend the coupler molecules in droplets of high boiling organic liquid, referred to as coupler solvent, which are dispersed in the desired layer of the photographic element, and thereby anchor the coupler in the layer and prevent it from diffusing to adjacent layers or out of the element during processing with various aqueous processing liquids. Thus, the needs for lower equivalent weight and sufficient organic ballast are at apparent cross-purposes.
Also, some characteristics of pyrazoloazole magenta-dye-forming couplers are significantly affected by the nature of any particular substituents that may be bonded to the coupling moieties at their coupling position. For example, it is known that the nature of such substituents can have a significant effect on how quickly and efficiently a coupling moiety can couple with oxidized developing agent at the coupling position to form a dye moiety, because such substituents must detach from the coupling position during the coupling reaction. Furthermore, after detachment from the coupling position, such substituents can remain in a photographic element along with the dye produced by the coupling reaction, and it is known that the nature of such detached substituents can then significantly affect the stability of the dye produced and can also significantly affect other components or activity in the photographic element, e.g., the rate of further development by developing agents.
There is therefore a continuing need for a new class of magenta-dye-forming couplers that can minimize the drawbacks described above, i.e., that are efficiently reactive with oxidized developing agent to form magenta dyes in silver halide photographic elements that yield relatively high levels of maximum density (Dmax) and contrast, that form magenta dyes having relatively high light stability, and that have relatively low equivalent weight, while at the same time having relatively high molecular weight to provide sufficient organic ballast for proper incorporation and anchoring in photographic element layers. It would also be desirable for such a new class of couplers to provide the flexibility to choose among various different substituents to have at the coupling position of the coupling moieties of such couplers, in order to be able to tailor the effects of such substitutents (effects such as described above) to meet particular needs in various photographic elements. Of course, the couplers should also exhibit all the other characteristics desirable for good photographic performance.